xref: /dflybsd-src/lib/libc/gen/dlsym.3 (revision 4a5f69f602439e4ddaa5c06d3f350823dac88dd0)

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.Dd February 22, 2018
.Dt DLSYM 3
.Os
.Sh NAME
.Nm dlsym ,
.Nm dlfunc
.Nd shared object symbol lookup function
.Sh LIBRARY
This function is not in a library.
It is included in every dynamically linked program automatically.
.Sh SYNOPSIS
.In dlfcn.h
.Ft void *
.Fn dlsym "void * restrict handle" "const char * restrict name"
.Ft dlfunc_t
.Fn dlfunc "void * restrict handle" "const char * restrict name"
.Sh DESCRIPTION
The
.Fn dlsym
function
returns the address binding of the symbol described in the null-terminated
character string
.Fa symbol ,
as it occurs in the shared object identified by
.Fa handle .
The symbols exported by objects added to the address space by
.Fn dlopen
can be accessed only through calls to
.Fn dlsym .
Such symbols do not supersede any definition of those symbols already present
in the address space when the object is loaded, nor are they available to
satisfy normal dynamic linking references.
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv NULL ,
it is interpreted as a reference to the executable or shared object
from which the call
is being made.
Thus a shared object can reference its own symbols.
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_DEFAULT ,
the search for the symbol follows the algorithm used for resolving
undefined symbols when objects are loaded.
The objects searched are
as follows, in the given order:
.Bl -enum
.It
The referencing object itself (or the object from which the call to
.Fn dlsym
is made), if that object was linked using the
.Fl Wsymbolic
option to
.Xr ld 1 .
.It
All objects loaded at program start-up.
.It
All objects loaded via
.Fn dlopen
with the
.Dv RTLD_GLOBAL
flag set in the
.Fa mode
argument.
.It
All objects loaded via
.Fn dlopen
which are in needed-object DAGs that also contain the referencing object.
.El
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_NEXT ,
then the search for the symbol is limited to the shared objects
which were loaded after the one issuing the call to
.Fn dlsym .
Thus, if the function is called from the main program, all
the shared libraries are searched.
If it is called from a shared library, all subsequent shared
libraries are searched.
.Dv RTLD_NEXT
is useful for implementing wrappers around library functions.
For example, a wrapper function
.Fn getpid
could access the
.Dq real
.Fn getpid
with
.Li dlsym(RTLD_NEXT, \&"getpid\&") .
(Actually, the
.Fn dlfunc
interface, below, should be used, since
.Fn getpid
is a function and not a data object.)
.Pp
If
.Fn dlsym
is called with the special
.Fa handle
.Dv RTLD_SELF ,
then the search for the symbol is limited to the shared object
issuing the call to
.Fn dlsym
and those shared objects which were loaded after it.
.Pp
The
.Fn dlfunc
function
implements all of the behavior of
.Fn dlsym ,
but has a return type which can be cast to a function pointer without
triggering compiler diagnostics.
(The
.Fn dlsym
function
returns a data pointer; in the C standard, conversions between
data and function pointer types are undefined.
Some compilers and code checkers warn about such casts.)
The precise return type of
.Fn dlfunc
is unspecified; applications must cast it to an appropriate function pointer
type.
.Sh NOTES
ELF executables need to be linked
using the
.Fl export-dynamic
option to
.Xr ld 1
for symbols defined in the executable to become visible to
.Fn dlsym .
.Sh RETURN VALUES
The
.Fn dlsym
and
.Fn dlfunc
functions
return the address of the symbol unless the symbol can not be found.
In this case, they return a null pointer and set an error condition
which may be queried with
.Fn dlerror .
.Sh EXAMPLES
The following program will obtain a pointer to the cosine function using
dlsym, and then it will use it to print out the value of cosine (2.0).
.Bd -literal
#include <dlfcn.h>
#include <stdlib.h>
#include <stdio.h>

int
main (int argc, char *argv[])
{
    void       *handle;
    double     (*func_cosine)(double x);

    /* open the system shared math library */
    handle = dlopen("libm.so", RTLD_LAZY);
    if (!handle) {
       fprintf (stderr, "%s\en", dlerror ());
       exit (EXIT_FAILURE);
    }

    /* get pointer to cosine function */
    func_cosine = dlsym (handle, "cos");
    if (func_cosine == NULL) {
       fprintf (stderr, "%s function not found\en", "cos");
       dlclose (handle);
       exit (EXIT_FAILURE);
    }

    /* Calculate and display the cosine of 2.0 */
    printf ("cosine of 2.0 = %f\en", func_cosine(2.0));
    dlclose (handle);

    exit(EXIT_SUCCESS);
}
.Ed
.Sh SEE ALSO
.Xr rtld 1 ,
.Xr dlfcn 3 ,
.Xr dlopen 3 ,
.Xr dlvsym 3